1. Field of the Invention
The present invention relates to a method and apparatus for cleaning drilling fluids and, more particularly, to such a method and apparatus wherein drilled solids on a screen separation device are contacted with an aqueous liquid from a tray disposed adjacent the screen.
2. Setting of the Invention
In drilling a well by rotary drilling, drilling fluid is continuously circulated from the surface through the drill string, through the bit, and up the annulus between the drill stringing and the borehole to the surface. The drilling fluid lubricates and cools the bit while drilling and removes drilled solids from the borehole. For the drilling fluid to perform satisfactorily, the viscosity and density of the drilling fluid must be maintained within certain limits.
A problem encountered while drilling is controlling the amount of drilled solids in the drilling fluid. The drilled solids may be bit cuttings and certain formation material. If permitted to accumulate in the drilling fluid, these drilled solids increase the viscosity and density of the drilling fluid which reduces the lubricating and cooling capabilities of the fluid, as well as possibly damaging some of the drilling equipment.
After exiting the wellbore, the drilling fluid may be passed over a screen separation device, such as a shale shaker or the like to separate a major portion of the drilled solids from the drilling fluid. The drilled solids which do not pass through the screen of the shale shaker are discarded. The other components of the drilling fluid, including liquids and smaller drilled solids, which pass through the screen are either directly returned to the drilling fluid system or are passed to additional drilling fluid cleaning devices.
One such additional cleaning device is a mud cleaner which consists of a plurality of centrifugal separation devices, such as hydrocyclones, and a screen separation device. The centrifugal separation devices remove a majority of the liquid components of the drilling fluid, such as water, bentonite and other chemicals, and are directly returned to the drilling fluid system. The drilled solids and dense components exit from the centrifugal separation devices and are introduced onto the screen separation device. The drilled solids cannot pass through the screens and are discarded. The remaining components, including liquids and silt, may be returned to the drilling fluid system.
In the event that a weighted drilling fluid is utilized in drilling the well, a certain portion of the weighting material may be accidentally discarded with the drilled solids while being treated in the additional cleaning devices. The weighting material tends to adhere to the drilled solids and is difficult to be removed therefrom. Water jets have been tried in the past to direct a spray of water onto the drilled solids while on the screen to aid in the separation of the weighting material from the drilled solids. The weighting material and the water pass through the screens. Further, the water passes through the screen and may be returned to the drilling fluid system along with the weighting material. The use of water in this manner has numerous disadvantages, including the fact that the drilling fluid system may become diluted with water which requires the addition of drilling fluid components to maintain the required drilling fluid characteristics. Further, the use of large amounts of water in remote, arid drilling regions is severely restricted and costly.